Process for making carbon agglomerates



Sept. 30, 1969 T. E. BAN 3,470,275

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United States Patent O U.S. Cl. 264-29 5 Claims ABSTRACT F THEDISCLOSURE This invention is in a process for producing a carbonagglomerate from raw finely divided coke material which comprisescarbonizing a burden of raw extrudate or compacted bodies of a mixtureof such coke with a tarry binder at elevated temperatures which reachinto the range of from 1600 F. to 2100 F. by passing hot gasestransversely to the direction of movement of the burden, and recyclingthe gases through the trevelling bed to cool the burden to an exittemperature of less than about 1000 F. and to reheat the gasses forrepassage through fresh burden to carbonize 0r garphitize the coke andyield a hard, baked, product which is suitable for use in makingnumerous carbon products, for example, electrodes.

This invention relates, as indicated, to a process for carbonizing andagglomerating relatively finely divided coke to yield a carbonaceousproduct which is more readily adapted for certain uses such as, forexample, the formation of electrodes, and particularly electrodes usefulin the production of aluminum metal. While this process will beillustrated with respect to lluid coke, it will be understood that othercokes, or mixtures thereof, such as petroleum coke or delayed coke, woodcoke, or coke derived from coal may be used as wel as a substitute forpart or all of the fluid coke.

The preparation 0f electrodes, for example, from fluid coke is describedin Patent 2,805,199, dated Sept. 3, 1957, which sets forth the method ofobtaining fluid coke and the properties of such a material. This type ofcoke is a specific example of a material which is useful in accordancewith the present invention.

For use in the manufacture of anodes for aluminum metal production, thedensity of the coke should be adjusted to be in the range of from about1.8 to about 1.95, preferably 1.87 to 1.92, and have a resistivity of20--30 103 ohm-inches. In general, this is accomplished by calcining thecoke, particularly fluid coke, at a tern perature in the range of from2000 to 2800 F. or higher. After the coke has been calcined to adjustits density to the desired range and its resistivity to within thedesired range, a portion of the coke is then ground so that from 20 to50 weight percent of the coke utilized in manufacturing the electrodeshas an average diameter of less than about 75 microns. As indicated inPatent 2,805,199, the grinding of the coke particles can be accomplishedin any conventional manner, and grindingmay precede the calcining stepbut preferably follows the calcining step.

It was found that although electrodes made in accordance with theteachings of Patent 2,805,199 were useful, it was desirable to improvethe properties of the electrodes by including in admixture with thefluid coke, a proportion of larger diameter coke which had also beencalcined, but had a particle size covering a very broad range of fromabout one inch in diameter to about 200 mesh, the proportion havingparticle sizes in this range being about 80 weight percent, with thebalance finer than 200 mesh. Carbon electrodes manufactured from such a3,470,275 Patented Sept. 30, 1969 ICC mixture of calcined delayed cokeand calcined uid coke are described in Patent 2,835,605, dated May 20,1958. Principal problems with electrodes made solely from uid cokeinclude dusting, premature breakdown or shredding of the electrode inthe aluminum bath with attendent increase in the electrode requirements,and the experiencinghof diiiiculty with short-circuiting in theelectrolyte bat The present process represents an improvement on priormethods of making electrodes from relatively finely divided coke, suchas fluid coke, whereby it is possible to convert relatively nely dividedcoke material into an agglomerate wherein the coke has the desireddensity and resistivity characteristics as above stated, and which isamendable to crushing and size reduction to yield particles having abroad spectrum of particle sizes such that the diiculties of dusting,premature breakdown or shredding, electrode economics, and processingdiiculties can be greatly minimized.

It has been found that relatively finely divided cokes may be admixedwith a carbonaceous binder in an amount conveniently ranging from about18 to 45 parts by weight per hundred parts coke. To determine theminimum amount of coal tar pitch or other binder required to adequatelybond the nely divided coke, a procedure such as that described in U.S.Patent No. 2,751,782, dated lune 26, 1956, may be used. By process theminimum pigment-volume concentration, or critical pigment-volumeconcentration may be accurately determined, the coke being considered asa solid particulate material or pigment dispersed in the pitch. Such adetermination will, of course, be made of an elevated temperature wherethe pitch is a liquid. The binders utilized for this purpose areconventional and include materials such as aromatic coal tar pitchbinders, for example, see U.S. Patent No, 2,683,107. These bindersgenerally have melting points within the range of from about 70 to 120C. and contain relatively small amounts of hydrogen, usually on theorder of about 5% or less. The concentration of benzene andnitrobenezene insoluble proportions represents preferably from about20%-35% and 5%-15%, respectively, of the binder. The coke is admixedwith the coal tar pitch at an elevated temperature to provide a uidmass, and this uid mass then normally extruded to produce cylindricalcompacted bodies or slugs about 1.5 inches long and having a diameter ofabout .75 inch. When the slugs are cooled to normal ambienttemperatures, they set up to a normally solid material and are theneasily handled. The coke which is utilized in forming these slugs is, asindicated above, usually calcined and desirably ground. In certaininstances, raw coke which is unground may be used and calcined at thetime it is carbonized. Instead of extruded slugs, other compacted bodiesof coke-binder mixture may be used, e.g., pellets, briquettes, pills,etc.

Thereafter, the compacted bodies are charged to a horizontally movingtraveling grate` Such traveling grates may be of conventional design,such as the straight line continuously traveling grate, or it may be ofthe circular type such as described in U.S. Patent 3,302,936, dated Feb.7, 1967. The raw extrudate slugs are charged to the traveling grate toform a relatively shallow bed having a depth generally less than aboutl2 inches, and preferably in the range of from about 4 to about 8 inchesin depth. The compacted bodies are baked by means of a draft of hotnonoxidizing gases passed transversely, preferably downwardly, throughthe burden composed of the compacted bodies or raw extrudate slugs in afirst zone. For the purposes of this invention gases containing lessthan about 5% by volume of oxygen are considered nonoxidizing orsubstantially neutral. In the cases of very reactive carbon forms,oxygen-free gases are desirably used. The gases may beneficially bereducing. The retention time of each incrementalv transverse section ofthe burden in the zone will depend, of course, upon the temperature andrate of passage of the gases through the bed, but in general, retentiontimes on the order from 5 to minutes are all that is necessary todevelop and move downwardly through the burden, a heat front whichreaches a maximum of from about 1600" F. to about 2000 F. Carbonizationof the binder together with some graphitization occurs along thetemperature front at about 1700 F. Volatiles are removed before thesetemperatures are acquired, and the binder of tar pyrolyticallydecomposes into a hard char matrix which bonds the uid coke particlesinto a coherent coked product. The gases are passed through the burdenat a rate which is generally within the range of from about 80 to about160 standard cubic feet of gas (standard conditions) per minute persquare foot of grate area, hereinafter abbreviated as s.c.f.m.

The burden then passes into a cooling or recuperation zone wherein theow of gases transversely to the movement of the burden is reversed, andthe gases which have by now been stripped of condensable materialsexternally of the burden and cooled, are recirculated through the burdenat substantially the same rate as in the previous zone to extract heattherefrom to condition the gases for repassage through the first zone toeffect the hardening, agglomeration, and carbonizing of the rawextrudate slugs. Usually, the draft in the first zone is downdraft, andthe draft in the recuperation zone is updraft. Thus, the temperaturegradient in the tirst zone proceeds downwardly and forwardly in thedirection of the continuous horizontal movement of the burden toward thegrates, and in the cooling or recuperation zone, the temperaturegradient proceeds upwardly and forwardly in the direction of movement ofthe burden until the final exit temperature of the burden is generallyless than 1=0O0 F. and usually ranges from a low of 150 F. to a maximumof about 800 F. To effect further cooling, water may be sprayed onto theburden as it discharges from the traveling grate. The retention time ofeach incremental transverse section of the burden in the cooling zone isabout equal to the retention time in the carbonizing zone.

The carbonizing and preheating draft from the cooling zone usuallycontains combustible matter and may be further heated through theintroduction of air to the systern which causes spontaneous ignition atthe temperatures involved, or it may be reheated through theintroduction of heated products of combustion from a torch, such as agas torch. Gases generated and admitted to the circuit in the course ofthe carbonization procedure necessitate continual venting of some of thegas stream. The vented gas may be routed through an afterburner forremoving smoke, combustible gases, and other entrained combustibleparticulate matter.

It is believed that while the extrudate is heated to a temperature of1700 F., the binder pyrolytically decomposes to form a high char matrixwhich lbonds with the uid coke particles to form a coherent cokedspecimen. The product when cooled is then crushed to give a structurewhich corresponds closely to that of delayed coke, i.e., having aparticle size over a broad spectrum of particle sizes and in the rangeof from about 1 inch in diameter to about 200 mesh, such a spectrumconstituting about 90% of the product, and the balance being finer than200 mesh. To reduce the spectrum to 80% of the product from 90%, iiuidcoke may be added. This product may then be utilized directly in themanufacture of electrodes according to known procedures, and when thestarting material has been iluid coke, such as described in theaforementioned Patent 2,805,199, the electrodes so produced areparticularly adapted for use in the electrolytic production of aluminumfrom alumina.

The invention may be better understood by having reference to theannexed drawings wherein: FIG. 1 is a diagrammatic and schematicrepresentation of a single stage indurating process embodying theprinciples of this invention. FIG. 2 is a diagrammatic and schematicrepresentation of a two stage indurating process embodying theprinciples of the present invention.

Reference may be had to the flow diagram of FIG. 1 which illustrates aspecific embodiment of the present invention utilizing a raw extrudateof a fluid petroleum coke such as set forth in the Patent 2,805,199, ora mixture of delayed coke and liuid coke as set forth in Patent2,835,605. In the example illustrated, the basis is 2,000 pounds of rawextrudate slugs 1.5" long by 0.75 in diameter which are introduced ontoa traveling grate machine of conventional structure to a bed depth ofabout four inches. The temperature at the point of introductionthroughout the entire depth of the bed is approximately 70 F., orambient temperature. As the burden moves into the tirst zone, thedirection of travel being from left to right as shown in FIG. 1, it iscontacted with heated gases having an average temperature of about 2000F. To effect the carbonization of one ton of raw extrudate, 7300 poundsof such gas having a heat content of 4.16 million B.t.u. are required,The rate of gas ow is about s.c.f.m./sq. foot of grate area.

The temperature of the burden is rapidly increased by the movement ofthe gases downwardly through the burden in a direction which istransverse to the direction of movement of the burden in a horizontalplane. There develops a heat front which can easily be detected withtemperature measuring devices disposed within the burden which proceedsalong a gradient moving diagonally, downwardly and forwardly through theburden until the temperatures of the uppermost layers of the rawextrudate are raised to temperatures at or above 1700 F. This gradientwill be identified as the carbonizing gradient and will represent animaginary line of temperatures progressing substantially diagonallydownwardly and forwardly through the burden at 1700 F. The temperaturesof the burden ahead of this gradient will, therefore, be in excess of1700 F. in the first zone, and the temperatures rearwardly of thegradient will be, therefore, less than 17.00 F. The retention time ofthe material within the rst zone averages about 7 minutes, and of thematerial within the second zone averages about 7 minutes, and thecarbonizing of the raw extrudate occurs throughout the entire crosssection of the burden. The grates reach a temperature desirably not inexcess of about 1000 F. In those cases where temperatures are apt toexceed about 1000 F., it may be desirable to provide a hearth layer ofinert material, or already carbonized slugs to serve as an insulatinglayer to protect the grates. Alternatively, the grates may desirably beliquid cooled. At the end of the process, the treated burden may bestripped from such a hearth layer, and the hearth layer recycled forreuse, or blended with the final product, and a portion thereof recycledas a hearth layer. The hearth layer technique in the art of conductinggas-solid phase reactions on a traveling grate with heated gases doesnot constitute a critical part of the present invention.

The temperatures of the gases exiting from the first zone average about900 F., and as will be noted, the weight of the gases has increased dueto the inclusion therein of volatile materials driven out of thecoke-binder composition. The heat content as indicated on the drawing isabout 1.7 million B.t.u. These gases are conducted conveniently througha scrubber, which may be a packed tower into which water is introducedfrom the upppermost portion thereof and allowed to trickle down over thepacking in a countercurrent fashion to the gases. This procedure servesto remove condensible and entrained materials, and to cool the gases tominimize damage to blower equipment utilized for return of the gases inan updraft manner through the burden as it passes through the coolingzone. The cooled gases serve also to control the temperature of thegrate and .prevent deleterious overheating. In the specific exampleillustrated, the amount of gases returned through the cooling zoneamounts to 7,385 pounds having a heat content of .15 million B.t.u. Theaverage temperature of the gases as they enter the grate area isapproximately 150 F.

As indicated above, the gases traverse the horizontally moving burden ina direction normal to the movement of the burden, and because of theincrease in the weight of the gases, a portion of the gases are ventedthrough an afterburner, entering the afterburner at a temperature ofabout 1100" F. and amounting to 2155 pounds. The heat content of thegases at this point is .63 million B.t.u. Air is introduced into theafterburner to elect ignition of combustible components of the gases,raise the temperature thereof, and consume the exit gases being passedthrough a heat exchanger prior to exhausting to the atmosphere.

Fresh air may be passed through the heat exchanger to preheat the sameprior to introduction into the system, or the heat may be recovered fromthe heat exchanges for use elsewhere in the plant. Thus, this portion isshown in dotted lines. In the normal course, ambient air in the amountof 1966 pounds may be pumped into the hood through which the gases arecirculating, and in the specific example shown, natural gas in theamount of 110 pounds having a heat content of 2.35 million B.t.u. may beadmixed and introduced into the recirculating gases within the hood tofurther increase the temperature and heat content to that point which isnecessary to effect the carbonization desired. The balance of the gasesfrom the cooling zone are, of course, recirculated through the hood asillustrated in FIG. 1, these gases now having a temperature of about1300* F. and amounting to 5280 pounds. The heat content is approximately1.84 million B.t.u.

As indicated in FIG. 1, there results from each ton of raw extrudateapplied to the traveling grate, a production amounting to about 1830pounds of hardened, carbonized slugs which are now ready to be crushedand utilized in the manufacture of electrodes as previously indicated.It will be noted that the amount of time required for conditioning thecoke has been reduced from a matter of days, in some cases, and hours inothers, to a matter of a relatively few minutes. The ability torecuperate heat content from the carbonized burden improves theeconomics of the treating process, and the independence frompredetermined mixtures of various kinds of coke in order to derivedesired properties in the final electrodes effects further economies.,Instead of utilizing coke or blends of various kinds of coke, asindicated above, there may also be used mixtures of coke with asphalticmaterials such as gilsonite or tar, for example, in Patent 3,025,229.

FIG. 2 in `the annexed drawings, illustrates a modification of the basicprocess which, instead of a single stage heating operation, as shown inFIG. 1, there is provided a two-stage heating system.

As indicated, there is provided a preheating zone in which gases at atemperature of about 900 F. are passed downwardly through the burdenwhere they become cooled by giving up the heat to the burden. The gasesexiting at the base of the burden are propelled by means of a fanthrough the terminal portion of the burden to cool the burden just priorto dumping from the traveling grate, said gases then exiting at atemperature of about 400 F. In like manner to the system shown in FIG.l, these gases are passed through an afterburner and heat exchangerprior to being exhausted to the atmosphere.

The central portion of the system shown in FIG. 2 correspondssubstantially to the single stage heating system shown in FIG. 1 withthe exception that the temperatures of the burden more rapidly reach thedesired 1700 F. gradient because of the preheating step. A portion ofthe gases exiting from the updraft cooling zone are also passed throughafterburner means to remove combustibles and to raise the temperature ofthe gases for introduction into the preheating zone. Inasmuch as thetemperature of the gases may be too high for use in a preheating zone,split stream cooling means are provided so that a portion of the gasesexiting from the afterburner are cooled and the resultant temperature ofthe gases as introduced into the preheating zone averages about 900 F.The modied process, as illustrated in FIG. 2, also results in theproduction of a flame front or temperature gradient proceedingdownwardly and forwardly through the burden and defning a line oftemperatures of approximately 1700 F. The temperatures of the burdenforward of this gradient line are in excess of 1700 F., and thetemperatures rearward of this line are less than 1700 F. Thetemperatures of the burden, as in the case of the burden in the systemillustrated in FIG. 1 are substantially above the volatilizationtemperature of any volatile components contained in the binder or in thepetroleum coke so that there is little or no internal condensation ofvolatilized organic material within the burden itself. Any condensationthat may occur, occurs externally of the burden and principally in thescrubber which is common to both modifications. The cooling zone islikewise analogous to the cooling zone in the single stage heatingsystem of FIG. 1 and effects, through a reversal of the direction offlow of the gases transversely through the burden, a reduction in thetemperature of the solid material constituting the burden, and anincrease in the temperature of the gases passing therethrough. Suchmovement of the gases through the burden results in a reversal of thedirection of the llame front or the temperature gradient of 1700 F. sothat such gradient now proceeds generally upwardly and forwardly. Thetemperatures of the burden to the rear of the gradient in the coolingzone are, therefore, above about 1700 F., and the temperatures ahead ofthe 1700 F. gradient are less than 1700 F. A final cooling zone isprovided for updraft cooling utilizing exhaust preheat gases which havebeen cooled to a relatively low temperature by the fresh burdenintroduced at ambient temperatures. Because some combustible materialmay be extracted from the burden in this zone, the gases exiting fromthe nal cooling zone are also passed through an afterburner and heatexchanger to exchange the heat content `thereof with incoming air or gaswhich is then introduced into the central carbonizing and recuperatingzone in the manner as shown in FIG. 1. Ignition of the combustiblecomponents of the gas within the central section circulating above theburden enables adjustment of the heat content of the circulating gasesto a proper point, i.e., about 2000 F. to promote establishment andpropagation of the llame front in the carbonizing zone downwardly towardthe grates, as indicated above.

There has thus been provided an improved agglomerating and carbonizingprocess whereby relatively nely divided particulate coke material may betreated rapidly and eflciently to produce a hardened carbonized productwhich, after crushing, is particularly useful as a material yfrom whichto produce electrodes, particularly electrodes for use inelectrometallurgical processes, such as the production of aluminum. Ingeneral, two types of electrodes are employed by the aluminum industryincluding (a) prebaked electrodes and (b) Soderberg self-bakingelectrodes. In the former process, a mixture comprising about 78% to 82%of a coke aggregate produced in accordance with the present inventionand from 13% to 22% of coal tar pitch is molded at pressures of about3,000-10,000 p.s.i. and extruded and then baked for periods up to 30days at l800 F. to 2400 F. Such preformed electrodes are then used inelectrolytic cells being slowly lowered into the molten alumina as theyare consumed.

The Soderberg process involves the continuous or intermittent additionof a coke-coal tar pitch paste into the top of the cell as the electrodecomponents in the lower part of the cell are consumed. In thisoperation, the paste represents a blend of about 70%-72% coke aggregatesuch as produced in accordance with the present invention and 25 %-'35of pitch. The cells usually operate at a temperature of 1700 F. to 1900"F. and electrodes are consumed at the rate of about 0.5 inch to 1.0 inchper day. The paste is baked into an electrode by the hot cell gases inthe period of time elapsing between when it is added at the top and thetime it is used. The net consumption of coke or electrode represents 0.4to 0.7 pound per pound of aluminum metal produced. It can be seen thatthe actual manner of fabricating the electrodes is not the essence ofthis invention. The production of an indurated coke aggregate suitablefor subsequent crushing to a desired broad range of particle sizes is aprincipal objective of this invention.

What is claimed is:

1. A process for carbonizing .and aggregating relatively nely dividedcoke comprising the steps of:

(a) charging compacted bodies of a coke-binder composition to atraveling grate to form a burden thereon, said burden having a depth offrom about 4" to about 12";

(b) moving the burden into a preheating zone;

(c) passing through the burden hot substantially nonoxidizing gases at atemperature between about 500 F. and 2000 F. transversely to thedirection of movement of the burden to preheat the burden and cool thegases;

(d) moving the preheated burden into a carbonizing zone;

(e) passing hot substantially nonoxidizing gases transversely to thedirection of movement of the burden to elevate the temperature of theadjacent burden toa carbonizing temperature of at least about 1700 F.progressively in a direction generally along and transversely to theburden to establish a carbonizing gradient through the burden;

(f) retaining each incremental transverse section of said burden in saidcarbonizing zone for a period of from about 5 minutes to about 15minutes;

(g) moving the burden into a cooling and recuperating zone;

(h) passing through the burden substantially nonoxidizing gases whichare cooler than the burden leaving the carbonizing zone transversely tothe direction of movement `of said burden to reduce the temperature ofthe burden and transfer a portion of the heat content of the burden tothe gases;

(i) recycling at least a portion of the gases from the recuperating zoneto the carbonizing zone for repassage through the burden;

(j) increasing the heat content of th egases recycled to the carbonizingzone;

(k) moving the burden into a final cooling zone;

(l) passing through ther burden substantially nonoxidizing gases issuingfrom the preheating zone and having a temperature -less than the burdenleaving the recuperating zone to further cool the brdenand raise thetemperature of the gases issuing from the burden in the final coolingzone; and

(m) recovering the heat from the gases issuing from the burden in thefinal cooling zone for utilization in heating the gases entering thepreheatng zone.

2. The process of claim 1 which is additionally characterized by thestep of removing condensible material from the gases issuing'from thecarbonizing zone externally of the burden. i

3. The process of claim 1 wherein the gases being recycled to thecarbonizing zone are heated to a temperature of at least about 2000 F.

4. The process of claim 3 wherein combustibles in the gases beingrecycled are contacted with air and ignited to raise the temperaturethereof and consume the oxygen content of the air to maintain therecycle gas substantially non-oxidizing.

5. The process of claim 1l wherein the rate of ilow of the gases throughthe burden in each zone is maintained in the range of from about toabout 160 standard cubic feet of gas per minute per square foot of greatarea.

References Cited UNITED STATES PATENTS 2,838,385 6/1958 Brown 44-233,009,863 11/ 1961 Angevine 202-26 3,010,882 11/1961 Barclay et al.'202-26 3,013,951 12/ 1961 Mansfield 201-27 3,077,439 2/1963 Shea et al.202-26 3,331,754 7/ 1967 Mansfield 201-39 FOREIGN PATENTS 23,941 12/1929 Australia.

DONALD I. ARNOLD, Primary Examiner JOHN H. MILLER, Assistant Examiner 2U.s. c1. x.R. zei- 21, 27, 29

